I'm going to stick my two-penneth in here even as a non expert. As has been said above, there is no clear "best car" as everyone will have their favourites. At the moment I have a YZ2 which I love, is robust and quick out of box, but there is no doubt that others are fast too.
One comment I will make is that KFs seem quick in dry but they also seem fragile. I recall one particular race meet at Batley earlier this summer with 6 cars in A final which were 3 KFs, 1 B5M and 2 YZ2s. Only 3 cars finished and none were KFs as all had a front left wishbone failure on the same corner / bump.
Losi seem to be very strong (I believe I've seen the term "tanks" used) but are not popular at Batley so I can't really comment on speed. I'm sure the new ones will be quick though.
Ultimately, go for what is popular at your club as you will be able to get set up help and urgent spares if needed at a race (so you don't end up going home due to a breakage) - particularly if going for a Schumacher as they seem to need a lot...
